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Migrating Monarchs
The North American monarch butterfly is a familiar companion to many American and Canadian childhoods. Boldly and handsomely marked in black, white and orange, they can be seen throughout the summer in much of the continental United States and southern Canada, near fields or waste ground, or anywhere their main food plant, milkweed, grows.
Few people, seeing a monarch flitting across their garden, would notice anything unusual about it. In fact, the North American monarch (the
scientific name is Danaus plexippus) has one of the most remarkable life cycles in the insect world. In common with many other species, several generations will be born each summer. The adult butterflies live only a few weeks, just long enough to mate and produce up to 400 eggs, always
deposited one at a time on milkweed plants.
This tough pale green plant exudes a gluey poisonous white sap that gives the family its name. In autumn each of the big oval seed pods release hundreds of little seeds with fluffy white thistledown-like parachutes. Monarch caterpillars feed on the leaves, somehow storing the toxins in their own bodies so that they will be unpalatable to predators. The adult butterflies retain this toxicity and the bold colouration is probably an advertisement to warn birds and other predators not to eat the poisonous mouthful. Other non-toxic butterflies have very similar markings, and lepidopterists (experts on butterflies and moths) have theorised that this mimicry has evolved to take advantage of the monarchs’protective chemistry: presumably birds seeing an almost identically coloured butterfly will assume that it, too, tastes bad.
However, the last generation of monarchs to be produced, as summer begins to turn into autumn, is different. Their bodies may be slightly
smaller than average, and for the most part they won’t mate or lay eggs. This is known as a state of “diapause”,when normal sexual processes are suspended. Barring accidents, they will live for eight or nine months. These monarchs must make an epic journey south, away from the harsh northern winter, and then return the following spring to produce the next year’s first generation.
Responding to cues such as the change in temperature, the shortening days and the dying back of the milkweed, the summer’s last generation of
monarchs begins to fly purposefully south and southwest, gradually coalescing into large groups that stream through parts of Texas, sometimes millions strong. They all head for a small area in the mountains of central Mexico: the Oyamel fir forest—a place, incidentally, that
none of them has ever seen before...
Migrating Monarchs
The North American monarch butterfly is a familiar companion to many American and Canadian childhoods. Boldly and handsomely marked in black, white and orange, they can be seen throughout the summer in much of the continental United States and southern Canada, near fields or waste ground, or anywhere their main food plant, milkweed, grows.
Few people, seeing a monarch flitting across their garden, would notice anything unusual about it. In fact, the North American monarch (the
scientific name is Danaus plexippus) has one of the most remarkable life cycles in the insect world. In common with many other species, several generations will be born each summer. The adult butterflies live only a few weeks, just long enough to mate and produce up to 400 eggs, always
deposited one at a time on milkweed plants.
This tough pale green plant exudes a gluey poisonous white sap that gives the family its name. In autumn each of the big oval seed pods release hundreds of little seeds with fluffy white thistledown-like parachutes. Monarch caterpillars feed on the leaves, somehow storing the toxins in their own bodies so that they will be unpalatable to predators. The adult butterflies retain this toxicity and the bold colouration is probably an advertisement to warn birds and other predators not to eat the poisonous mouthful. Other non-toxic butterflies have very similar markings, and lepidopterists (experts on butterflies and moths) have theorised that this mimicry has evolved to take advantage of the monarchs’protective chemistry: presumably birds seeing an almost identically coloured butterfly will assume that it, too, tastes bad.
However, the last generation of monarchs to be produced, as summer begins to turn into autumn, is different. Their bodies may be slightly
smaller than average, and for the most part they won’t mate or lay eggs. This is known as a state of “diapause”,when normal sexual processes are suspended. Barring accidents, they will live for eight or nine months. These monarchs must make an epic journey south, away from the harsh northern winter, and then return the following spring to produce the next year’s first generation.
Responding to cues such as the change in temperature, the shortening days and the dying back of the milkweed, the summer’s last generation of
monarchs begins to fly purposefully south and southwest, gradually coalescing into large groups that stream through parts of Texas, sometimes millions strong. They all head for a small area in the mountains of central Mexico: the Oyamel fir forest—a place, incidentally, that
none of them has ever seen before...
Photo credits, L to R: Tiger ©worldswwildlifewonders/Shutterstock.com; Monarch butterfly colony © Charles Shapiro/Shutterstock.com; Western diamondback rattlesnake © Audrey Snider-Bell/Shutterstock.com